Fair Value Measurements (Assets Measured At Fair Value Using Significant Unobservable Inputs) (Detail) - Level 3 - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
Mortgage-backed securities
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Balance- Beginning of Period, Assets	$ 61	$ 78
Transfers Into Level 3, Asset	0	1
Transfers out of Level 3, Assets	0	0
Asset, Net realized Gain (Loss) Included in Other Comprehensive Income (Loss)	0	0
Net Realized Gains/ (Losses), Assets	0	0
Purchases, Assets	18	4
Sales, Assets	0	0
Settlements, Assets	(1)	0
Balance-End of Period, Assets	78	83
Fair Value, Assets Measured on Recurring Basis, Change in Unrealized Gain (Loss)	0	0
Corporate securities
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Balance- Beginning of Period, Assets	1,299	1,037
Transfers Into Level 3, Asset	5	0
Transfers out of Level 3, Assets	0	(10)
Asset, Net realized Gain (Loss) Included in Other Comprehensive Income (Loss)	4	(3)
Net Realized Gains/ (Losses), Assets	1	0
Purchases, Assets	128	139	[1]
Sales, Assets	(37)	(51)
Settlements, Assets	(58)	(39)
Balance-End of Period, Assets	1,342	1,073
Fair Value, Assets Measured on Recurring Basis, Change in Unrealized Gain (Loss)	0	0
Foreign
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Balance- Beginning of Period, Assets	345	93
Transfers Into Level 3, Asset	3	7
Transfers out of Level 3, Assets	(15)	0
Asset, Net realized Gain (Loss) Included in Other Comprehensive Income (Loss)	6	9
Net Realized Gains/ (Losses), Assets	(1)	0
Purchases, Assets	53	87
Sales, Assets	(5)	(19)
Settlements, Assets	(26)	(1)
Balance-End of Period, Assets	360	176
Fair Value, Assets Measured on Recurring Basis, Change in Unrealized Gain (Loss)	0	0
Equity Securities [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Balance- Beginning of Period, Assets	57	44
Transfers Into Level 3, Asset	0	0
Transfers out of Level 3, Assets	0	0
Asset, Net realized Gain (Loss) Included in Other Comprehensive Income (Loss)	1	1
Net Realized Gains/ (Losses), Assets	(2)	2
Purchases, Assets	9	17
Sales, Assets	(10)	0
Settlements, Assets	0	0
Balance-End of Period, Assets	55	64
Fair Value, Assets Measured on Recurring Basis, Change in Unrealized Gain (Loss)	(1)	0
Short-term Investments [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Balance- Beginning of Period, Assets	1	0
Transfers Into Level 3, Asset	0	5
Transfers out of Level 3, Assets	0	0
Asset, Net realized Gain (Loss) Included in Other Comprehensive Income (Loss)	0	0
Net Realized Gains/ (Losses), Assets	0	0
Purchases, Assets	0	8
Sales, Assets	0	0
Settlements, Assets	(1)	(1)
Balance-End of Period, Assets	0	12
Fair Value, Assets Measured on Recurring Basis, Change in Unrealized Gain (Loss)		0
Other Long-term Investments [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Balance- Beginning of Period, Assets	11	263
Transfers Into Level 3, Asset	0	0
Transfers out of Level 3, Assets	0	0
Asset, Net realized Gain (Loss) Included in Other Comprehensive Income (Loss)	0	2
Net Realized Gains/ (Losses), Assets	0	0
Purchases, Assets	0	14
Sales, Assets	0	0
Settlements, Assets	0	(9)
Balance-End of Period, Assets	11	270
Fair Value, Assets Measured on Recurring Basis, Change in Unrealized Gain (Loss)	$ 0	$ 0

[1] Purchases in Level 3 primarily consist of privately-placed fixed income securities.